The present invention relates to circuitry for a selective push-pull amplifier.
Push-pull amplifiers comprise two amplifier triple poles with the same operating voltage polarity, whose control electrodes are controlled in phase opposition and whose output currents are combined by means of an output inductance, the latter simultaneously serving as the operating voltage supply for the amplifier triple poles. The term "two amplifier triple poles with the same operating voltage polarity" is understood to mean, e.g. two electron tubes, which must always be operated with a positive anode voltage. Further examples are two pnp-transistors, which are both operated with negative collector-emitter voltage, or two npn-transistors, which are both operated with positive collector-emitter voltage. All other electronic amplifier elements, such as e.g. the different types of field effect transistors are to be considered in a similar way. Due to the same operating voltage polarity of the two amplifier triple poles, the operating currents thereof also have the same polarity. Thus, they must be combined in phase opposition at the amplifier output, in order to superimpose them in the correct phase position for the load. This is brought about by an output inductance or output transformer which, in per se known manner, brings about a magnetic combination of the output currents of the two amplifier triple poles. In per se known manner, the operating voltage is supplied to the two amplifier triple poles by a centre tap at the output inductance or in primary winding of the output transformer.
The amplifier controls can be operated in each of the three known basic circuits. This would be the emitter-base and collector circuit in the case of a bipolar transistor. In each of these circuits is defined a control electrode (e.g. the base in the bipolar transistor emitter circuit) and an output electrode (e.g. the collector in the bipolar transistor emitter circuit) of the amplifier triple pole. Any known variant of the three basic circuits is conceivable for the amplifier triple poles, such as e.g. the emitter circuit with emitter resistance to current feedback.
Amplifiers are generally classified according to the control mode of their triple poles (e.g. tubes, transistors, FET's). The literature (cf. e.g. MEINKE-GUNDLACH, Taschenbuch der Hochfrequenztechnik, 3rd edition, Springer-Verlag: chapter O, transmitter amplifiers and neutralization) discloses A, B and C amplifiers. These classifications apply to wide-band amplifiers and selective (narrow-band) amplifiers both in single-ended and push-pull operation (cf. e.g. H. SCHRODER, Elektrische Nachrichtentechnik, Verlag fur Radio-Foto-Kinotechnik, Berlin: vol. II, chapter BIII/9, Push-pull circuits). The following comments are limited to push-pull amplifiers according to the preamble of claim 1.
The three amplifier classes differ fundamentally in their behaviour mainly with respect to the efficiency attainable and the nonlinear distortions which occur.
In the case of A amplifiers the efficiency .eta. is always below 50%, but they have the lowest nonlinear distortions. The low efficiency can be attributed to the fact that the amplifier triple poles carry current at all times and consequently there is always a power dissipation therein.
However, in B amplifiers a half-wave control is used, so that each triple pole is alternately free from current for a half-cycle of the instantaneous frequency. At these times, in which there is only voltage at the particular amplifier triple pole, but no current flows through it, no power dissipation is produced therein. Thus, the efficiency .eta. can be increased to max. 78.5%. However, compared with the A amplifier, the B amplifier has higher nonlinear distortions.
Finally, in the C amplifier, the control of the amplifier triple poles is limited to fractions of a half-cycle of the instantaneous frequency. Thus, the current-carrying times for the amplifier triple poles are further reduced compared with B amplifiers and the power dissipation in the triple poles is further reduced. However, in the now very short current-carrying phases, the amplifier triple poles must carry much higher currents than in the case of A or B amplifiers. In the extreme case, the current-carrying phase of the amplifier triple poles is reduced to zero. The efficiency can then become 100% in theory, but it will be necessary for the amplifier triple poles to supply infinitely high current pulses as in infinitely short time. These extreme values cannot be achieved in practice.
Thus, in the case of C amplifiers, each move towards a 100% efficiency must be bought with a great increase in the peak currents in the amplifier triple poles compared with the output currents to be effectively supplied by the amplifier. This requires greatly overdimensioned components which, due to the necessary short switching times, are usually very expensive and do not permit an economically favourable solution.
In addition, the harmonic frequency proportions increase in the current pulses with decreasing time and consequently increasing amplitude. Thus, in the C amplifier, an efficiency increase also leads to a further increase in the nonlinear distortions in the form of subsidiary waves or rising distortion factor. This effect can only be reduced by the increased filtering out of the harmonics. However, on increasing the filtering action, e.g. through reducing the band width, the filter losses increase, so that the efficiency of the circuit again drops.
Therefore the known A, B and C amplifiers are not in a position to simultaneously bring about high efficiency at minimum distortion. However, many applications exist where both requirements must be simultaneously fulfilled. For example, in the case of transmitter amplifiers, the minimum possible nonlinear distortions are required, due to the high subsidiary wave damping required in the case of transmitters. However, the efficiency must also be as high as possible, so that with a high transmission power, the dissipation remains as low as possible. As both requirements cannot be jointly fulfilled by any of the known amplifier circuits, it is generally necessary to accept a low efficiency in order to achieve an adequately low nonlinear distortion level.